Forced ignition

It is possible to conclude from the preceding that the study of the laws governing thermal explosions will increase understanding of the phenomena controlling the spontaneous ignition of combustible mixtures and forced ignition in general. 

The theory of spontaneous ignition can easily be understood by examining how combustion starts in a closed system, while forced ignition is discussed in an open system.13... 

The first visible sign of combustion is the ignition. It may arise from self-ignition when the combustible system is ignited by virtue of the spontaneous internal exothermic processes. In the other case, forced ignition occurs when the combustible system is ignited by an external ignition source. 

Fig. 2.5 Forced ignition temperature of a gaseous mixture brought about by a hot walF Tq is the temperature of the gaseous mixture Tg, are wall temperatures Curves /, 2, 3 temperature plots of the inert gas Curves 1 2 3 temperature plots of the combustible gas...

This is the reason why the published self-ignition and forced ignition temperatures of the same material show such a large scatter the differences can be as great as 200 °C. 

The Priifstrahler (test irradiator) instrument (Pyro-Werk, Hannover, FRG) is suitable for this purpose (Fig. 3.26). Its ellipsoidal space with a reflective inside surface contains electric resistance ribbons at one focus and the 70 mm x 70 mm specimen at the other. The intensity of heat radiation can be regulated between 0 and 6 W/cm, focused at the surface of the specimen on a circular area of 25 cm. 35 mm above the specimen, a spark gap serves to ignite the decomposition products if forced ignition is necessary. 

Fig. 11.41 The diagram of experiments using forced ignition of a hydrogen diffusion flame [42] 1 -nozzle 2 - gas analyzer 3 -spark-gap 4 - flame 5 -methane concentration detector 6 - spark control 7 - high-speed camera 8 -pressure transducer 9 -anemometer 10- 98.5% H2 + 1.5% CH4 mixture at / = 40 MPa...

---